This invention relates to a monitoring and controlling system for a nuclear reactor control rod drive system and, more particularly, a system for preventing erroneous operation of control rods due to erroneous operation of control rod driving means or erroneous manual operation of the control rod.
The control rod is used to maintain uniform output distribution in the nuclear reactor and control the reactivity. In most reactors, the reactivity is controlled in a manner that control rods made of neutron absorbing materials are moved into and out of a reactor core. One of the most important requirements in the reactor control is to secure a sufficient safety in fission reaction. For this, continuous care must fully be taken for the control system of the control rods.
In the prior art, a control rod worth minimizer and a rod block monitor have been used for monitoring drive of the control rods.
The control rod worth minimizer monitors movements of the control rods included in a group selected from all the control rods distributed in the reactor core, and blocks the movement of a control rod which is not included in the selected group when such a control rod is erroneously operated before the control rods of the selected group are withdrawn to a predetermined position. The rod worth minimizer serves to monitor control rods operation by an operator on the basis of a predetermined withdrawal/insertion sequence (CR sequence) of the control rods which is previously stored in a computer. Monitoring by the control rod worth minimizer is made for a plurality of group in a predetermined order. Thus, the minimizer with such a function makes uniform the reactivity suppressing abilities of a number of the control rods which are inserted in the reactor core, preventing undesirable withdrawal of a specific control rod which results in unexpected increase of the reactivity. The ability is called a control rod value. The minimizer usually operates at the incipient stage of the reactor starting operation, that is, in the range of power levels of 0% to 20-30% of the rated power. In higher power levels exceeding this range, finer control is required for the control rod operation. However, it is very difficult to change the CR sequence stored in the minimizer. In the higher power levels, it is also difficult to analytically prepare a CR sequence. In other words, it is undesirable from a view point of operation flexibility that the CR sequence once prepared and stored restricts all the operations related. It is for this reason that the rod block monitor is used for monitoring the control rod operation at the higher power level. The rod block monitor monitors a state of the power level of a reactor core by using an amount of neutrons detected by neutron measuring devices. The rod block monitor generates a signal to block the operation of a control rod at the time that an output of the neutron measuring device becomes equal with or larger than a predetermined value so as to block the operation of the control rod. However, the neutron measuring device detects also fluctuation of neutron quantity arising from variation of neutrons generated in the reactor. Therefore, it must be prevented to produce a signal for blocking the operation of the control rod if the detected quantity of neutrons is within a predetermined fluctuation or tolerance of the neutrons. For this reason, a set value for producing the block signal is set up at a power level 5 to 10% higher than the rated power output of the nuclear reactor. This brings about a problem that the blocking of the control rod operation is delayed.
At present, stability analysis of the nuclear reactor proves that when both the control rod value minimizer and the rod block monitor are applied for the control rod operation, safety of the nuclear reactor can be ensured even if a control rod is erroneously withdrawn due to the erroneous operation of associated apparatus or by an operator. However, if the erroneous operation of the control rod may be blocked at the initial stage of the erroneous operation, admitted range of the operation of the nuclear reactor is extended to increase the reactor power output.